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Why isn't there an `at(At.list(), 12)` that allows to pinpoint on element #12 in a list in Optics? Maybe there's another way to do this that I'm missing?

there's `at` and `index` in the `arrow.optics.dsl` package

Oh ...
```index(Index.list(), 0)```
that wasn't so obvious. Why not just `at(At.list(), 0)` just like there is for `At.set()` and `At.map()`?

And that `index(..)` allows modifying that specific entry?

`At` and `Index` are quite different, `At` works with a `Lens` while `Index` works with an `Optional`.

`At` works quite different than what you're describing for `Map` and `Set`.

`At` for a `Set&lt;A&gt;` looks whether an `A` is present, and checking if the `Set` `contains` that `A` or it allows you to remove it from the `Set`. (`Lens&lt;Set&lt;A&gt;, Boolean&gt;`).

`At` for `Map&lt;K, V&gt;` looks wheter a value is present at `K`, and allows retrieving or removing that value using `Option&lt;V&gt;`. So setting `None` removes it, and setting `Some(V)` updates it. (`Lens&lt;Map&lt;K, V&gt;, Option&lt;V&gt;&gt;`).

So that is extremely different from what working over an `Index` does.

Thanks for the explanation! So `index(..)` is saying, if it's there, get/set/transform it, if not, nothing happens (and return none?). Whereas `at(...)` is pinpoint at an element, if it's there, get/modify/remove it, if not, create it?

I still don't understand why Arrow wouldn't provide a specialized `index(Index.listBy { it.id }, FooId(1))`  for fetching by map key or some property in an element in a Set...

But maybe I'm starting to understand why `at(...)` is not implemented for a list (by index at least), because we could get to invalid states like trying to remove an element in the middle of a list (thereby changing all the indices). But it could still be useful to have an `at(At.listBy { it.id }, 1)` where `id` is a property on the elements in the list...